The "lip" or "pinch weld" needs to be brought out as well because if you bring out the body with the triangle shaped metal as in my drawing the lip is going to be covered up. A portion of it will be gone or near gone, that is why I said to bring the lip over. It is super, super easy, and it is an important part of the windshield mounting and sealing.
DBM, the plug welds are NOT for strength, it IS to only hold the metal in place prior to the welding of the seam.
This is a test coupon from a welding test on aluminum. It shows how a buttweld with backing works. You are welding TO the backing, that is where you want your wire to hit down into the backing. Then you bring the weld up onto the panels you are welding together. To past the ICAR or AWS (American Welding Society) you need to fully penetrate that backing. The backside of the backing needs to show melt thru the entire length of the weld.
I agree that they should be plug welded only because you are very right that it IS a form of reinforcement and if the butt weld is not up to par, the plug welds are adding a little more strength. But honestly, if the butt weld is done properly that plug weld is doing nothing but going along for the ride.
I personally spend a lot of time fitting that backing, it doesn't look like the one on the Model A photo I posted. Now, I have never used this method on a chop, I haven't done a chop or body section or anything like that since I learning about the butt weld with backing, plug weld, flange weld and all the other forms of joining two sheets of metal together. I may have "done" them, but I didn't "know" about them if that makes any sense.
I didn't start using them on a regular basis until ICAR (click here for ICAR)
and manufacturer training. So I used them every single day in collision repair being they are the "standard of the industry" ways of "sectioning" panels.
And in those cases, I cut the backing from the old panel I just cut off to discard. The part you cut off the top to lower it of course is a perfect source for our backing! So why not cut an inch off this piece and use the whole darn thing for you backing, not just "patches" like on that Model A body.
You cut off an inch from the piece, trim the ends of course because you can't have it overlapping at the pinch weld (window lip), so you cut that portion off. But other than that or any other places where you need trim it. You leave the curves and body lines there, I mean, you HAVE THEM right there in front of you in that piece you cut off to lower the top! You don't have to make them, they are right there in front of you!
So, you take that one inch piece that fits perfectly (sometimes you need to reform it a little because it IS after all going on the inside of a curve and may need to be "shortened" by moving a bend to fit) along the entire weld. You butt weld it as in the photo above and end up with a perfectly strong joint. Where I agree is certainly with the plug welds. They are usually smaller (1/4" is plenty) and not as many as that test panel photo, that was for weld testing or practice or something, you don't need that many plug welds to hold that little strip in. But I DO plug weld it in because I want it perfectly placed and not moving around when I weld the seam. But that is just me and the way the "standard" is taught in the collision industry. There are MANY, MANY places where they are used in late model cars that are double panel like roof pillars and rockers where there is no way you can hold it, it MUST be plug welded.
But on something like this truck, sure if you wanted to you could clamp it in place when you weld your seam, in this case sure, if that is what you want to do and there is room, clamp it in. I personally still plug weld because it has became the norm for me.